The biological effects of electromagnetic radiation to human beings can be divided into two categories, ionizing and non-ionizing radiation. The first, ionizing radiation, is related to cosmic and x-ray wavelengths and to nuclear radiation. The second, non-ionizing radiation is related to ultraviolet, visible, infrared, microwave, and radio wavelengths. Image and video projection devices works within the visible spectrum of the light, therefore, non-ionizing hazard has to be investigated and avoided while running the device.
Biological effects of non-ionizing radiation are dependent on the spectral region of the radiation (wavelength) and the duration of the exposure to the radiation. Furthermore, the damage to the eyes and skin is dependent on whether there was a single exposure (acute) or daily exposure (chronic) to the radiation.
The eye, are generally considered to be the organ of the body which is most susceptible to damage by radiation. The parts of the eye that can be affected by radiation are the cornea, lens, eye fluid, and retina. Different light radiation affects the individual eye parts. The damage to any of the parts occurs when the light is absorbed by the parts. The damage that takes place is dependent on the ranges of the exposure levels and the time of exposure.
Visible wavelengths of radiated frequency range from 390 nm to 750 nm, those wavelengths are generally refracted by the cornea and absorbed by the retina.
The Maximum Permissible Exposure (MPE) of the eye to visible radiations within 400-700 nm wavelengths is about 0.001 W/cm2 for an exposure time of 10 seconds. Therefore a method to prevent damage is needed to maintain an exposure lower than the authorized MPE.
In addition, the eye is also sensitive to other wavelength that can induce severe damage, in the ultraviolet and infrared and therefore for device using such wavelength, a method for preventing damage is also required.
In the past years, many different types of electronic devices using laser units in order to perform one or more technical functions have been developed. Micro-projections systems are among these devices. With the growing demand for laser diode for various applications such as telecom and laser pointer device, the eye safety issue for human became an issue and was mainly handled by different methods. The simplest method was to use stickers placed at the tip of the laser pointer device and warning the user to avoid any direct eye illumination with the laser. Another method was to develop a specific driving electronics in order to avoid any peak current in the laser diode if electrical failure occurs or to completely switch off the current in the laser diode above a certain current level.
More advanced technique where described in past for eye laser safety, using CCD detector coupled with the laser source. The CCD detector detects the motion of an object or a person in its vision field and sends a signal to stop the laser in the case of a movement.
Other technique use motion sensor, such as accelerometers or/and gyroscope, coupled with the projection system and sense motion of the projector itself, and then send a signal to the laser source to either switch off or lower the intensity. Other techniques also have been tested in the past using capacitive sensor where one electrode is placed in the measurement tool and the human body acts as a second electrode. The human presence is then sensed by the created voltage shift between these two electrodes.
A problem of these laser safety techniques, for the specific laser projection application, is that none of these existing techniques are completely efficient, as the existing solutions do not prevent eye damage if the user switches on the projector while he looks directly toward the light source and while not moving. This specific aspect and possible risk damaging the eye, is possibly one of the key stoppers for the use of such laser projector by a large number of people and especially by children. Moreover, most of these techniques are complex and expensive.
Another problem of the existing techniques to prevent eye damage is that the use of a CCD detector or capacitive detection do not allow having a directional sensing and is typically much larger that the field of projection of moving object. The result is that a moving object placed outside the field of projection, and then fully safe in terms of eye safety, will be sensed by the CCD detector and will either stop of lower the projection intensity of the laser. For hand-held application of laser projector, this working behaviour, even though allowing eye safety apparatus is therefore not adapted to normal operation and use. Indeed the user should be able to use the laser projector in a crowed place and should be able to project while maintaining the projector in its hand, and during any motion.
A further problem of the eye safety technique while using laser source, is that the use of a CCD detector or an external motion sensor further increase the complexity of the overall laser-based projection system by adding a different technology to the technology initially used for the projection purpose.
A known type micro-projection systems based on Micro-Electro-Mechanical System (MEMS) is presented in FIG. 1A, where two MEMS scanning mirrors 102 and 103 are reflecting a laser light source 101 is order to project a two dimensional image on a target screen 104. Other projection system, presented in FIG. 1B and based on matrix of a large number of individual addressable pixels 105, either based on either MEMS technology or Liquid Crystal on Silicon could also use laser source to project image.
A complete architecture for laser-based colour projection using two One-Degree-Of-Freedom (1 DOF) MEMS scanning mirrors is presented in FIG. 2. The laser beams 200 are combined using a beam combiner 201 optic device and the resulting beam is entering a beam splitter 202 and is deflected by the two MEMS scanning mirror 203 to project a two dimensional image. However, in existing projection systems, there is no complete safety system that can avoid eye damage when the eye is within the field of projection above the MPE limit, as presented in FIG. 3.
Other electronic devices using self-mixing technique are also known. For instance, WO2005/106634 discloses an apparatus for handling sheet material or an optical input device, which employs a relative movement sensor utilizing the so-called “self-mixing” effect of a laser diode. A band pass filter is provided for filtering the electric signal resulting from measurement of the electric signal to reduce or substantially eliminate the effects of both the low frequency carrier signal and the high frequency noise present in such a signal. As a result, the precision of the laser self-mixing translation measurements is significantly improved.
U.S. Pat. No. 6,233,045 relates to a self-mixing sensor usable for remotely measuring speed, vibrations, range, and length provided in a manner making the device practical for economic implementation while retaining accuracy. In one embodiment, the device is configured to avoid mode hopping, such as by providing for relatively high loss for all modes other than the desired mode. Preferably this is accomplished by utilizing laser types that have a high degree of side mode suppression, such as DFB lasers or through active or passive control of the amount of light permitted to re-enter the laser.
However, these devices are of no use to provide micro-projection system or methods.
WO2007/062154 relates to a method for compensating non-uniformities of a projection surface in a front projection display. The measured properties of the surface are used to provide a screen compensation bitmap or a screen compensation convolution table. To obtain the screen compensation map, the method involves measurement of brightness for each pixel and storing the related values in the map. The compensated image is obtained in modifying the grayscale values of the pixels in the video image according to corresponding values in the screen compensation map to produce a compensated video image signal.
According to this method, the applied correction directly depends on the projected image. Thus, if an image showing an irregular surface such as a cushion or non ironed clothes or sheets, etc, is displayed, the projection system will use the method to compensate the image as if the irregularities where caused by the projection surface. Moreover, the correction depends on the image taken by an additional camera or device which is not perfectly aligned with the projection system, which adds to the cost and complexity, and creates parallax errors. This device is of no use for eye-safety.
US2009/0147272 describes a proximity detection method for controlling of an imaging device. A proximity detector is capable of estimating the distance from an object to the projector. If an object is detected within a minimum distance, the projector operation may be altered, for example to cause the projector to turn off or to reduce the intensity of the emitted light below a selected range. In a first embodiment, the detection module uses periphery detection to detect the presence of an object in front of the projector. The proximity detector projects nearly collimated beams of infrared light to create spots that are placed around a display region projected by a projector. The reflected beams are then detected by a linear array of sensors, which detects reflection of the beams within a detection cone. In a second embodiment, a detection module uses triangulation based distance estimation to detect the presence of an object in front of the projector. Such a system involves specific infrared emitters and detectors in addition to the standard projection material. Distance data involve only few points, therefore limiting accuracy and potential other uses of the data. Again, the distance measuring system is not aligned with the projection system, creating parallax errors.